java.util.concurrent

Class CopyOnWriteArraySet<E>

  • Type Parameters:
    E - the type of elements held in this collection
    All Implemented Interfaces:
    Serializable, Iterable<E>, Collection<E>, Set<E> 

    public class CopyOnWriteArraySet<E>
extends AbstractSet<E>
implements Serializable
    A Set that uses an internal CopyOnWriteArrayList for all of its operations. Thus, it shares the same basic properties:
    • It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
    • It is thread-safe.
    • Mutative operations (add, set, remove, etc.) are expensive since they usually entail copying the entire underlying array.
    • Iterators do not support the mutative remove operation.
    • Traversal via iterators is fast and cannot encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

    Sample Usage. The following code sketch uses a copy-on-write set to maintain a set of Handler objects that perform some action upon state updates. 

      class Handler { void handle(); ... } class X { private final CopyOnWriteArraySet<Handler> handlers = new CopyOnWriteArraySet<Handler>(); public void addHandler(Handler h) { handlers.add(h); } private long internalState; private synchronized void changeState() { internalState = ...; } public void update() { changeState(); for (Handler handler : handlers) handler.handle(); } }

    This class is a member of the Java Collections Framework.

    Since:
    1.5
    See Also:
    CopyOnWriteArrayList, Serialized Form

    • Method Summary

      Modifier and Type Method and Description
      boolean add(E e)
      Adds the specified element to this set if it is not already present.
      boolean addAll(Collection<? extends E> c)
      Adds all of the elements in the specified collection to this set if they're not already present.
      void clear()
      Removes all of the elements from this set.
      boolean contains(Object o)
      Returns true if this set contains the specified element.
      boolean containsAll(Collection<?> c)
      Returns true if this set contains all of the elements of the specified collection.
      boolean equals(Object o)
      Compares the specified object with this set for equality.
      void forEach(Consumer<? super E> action)
      Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
      boolean isEmpty()
      Returns true if this set contains no elements.
      Iterator<E> iterator()
      Returns an iterator over the elements contained in this set in the order in which these elements were added.
      boolean remove(Object o)
      Removes the specified element from this set if it is present.
      boolean removeAll(Collection<?> c)
      Removes from this set all of its elements that are contained in the specified collection.
      boolean removeIf(Predicate<? super E> filter)
      Removes all of the elements of this collection that satisfy the given predicate.
      boolean retainAll(Collection<?> c)
      Retains only the elements in this set that are contained in the specified collection.
      int size()
      Returns the number of elements in this set.
      Spliterator<E> spliterator()
      Returns a Spliterator over the elements in this set in the order in which these elements were added.
      Object[] toArray()
      Returns an array containing all of the elements in this set.
      <T> T[] toArray(T[] a)
      Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.

    • Constructor Detail

      • CopyOnWriteArraySet

        public CopyOnWriteArraySet()
        Creates an empty set.

      • CopyOnWriteArraySet

        public CopyOnWriteArraySet(Collection<? extends E> c)
        Creates a set containing all of the elements of the specified collection.
        Parameters:
        c - the collection of elements to initially contain
        Throws:
        NullPointerException - if the specified collection is null

    • Method Detail

      • size

        public int size()
        Returns the number of elements in this set.
        Specified by:
        size in interface  Collection<E>
        Specified by:
        size in interface  Set<E>
        Specified by:
        size in class  AbstractCollection<E>
        Returns:
        the number of elements in this set

      • isEmpty

        public boolean isEmpty()
        Returns true if this set contains no elements.
        Specified by:
        isEmpty in interface  Collection<E>
        Specified by:
        isEmpty in interface  Set<E>
        Overrides:
        isEmpty in class  AbstractCollection<E>
        Returns:
        true if this set contains no elements

      • contains

        public boolean contains(Object o)
        Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)).
        Specified by:
        contains in interface  Collection<E>
        Specified by:
        contains in interface  Set<E>
        Overrides:
        contains in class  AbstractCollection<E>
        Parameters:
        o - element whose presence in this set is to be tested
        Returns:
        true if this set contains the specified element

      • toArray

        public Object[] toArray()
        Returns an array containing all of the elements in this set. If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

        The returned array will be "safe" in that no references to it are maintained by this set. (In other words, this method must allocate a new array even if this set is backed by an array). The caller is thus free to modify the returned array.

        This method acts as bridge between array-based and collection-based APIs.

        Specified by:
        toArray in interface  Collection<E>
        Specified by:
        toArray in interface  Set<E>
        Overrides:
        toArray in class  AbstractCollection<E>
        Returns:
        an array containing all the elements in this set

      • toArray

        public <T> T[] toArray(T[] a)
        Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array. If the set fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this set.

        If this set fits in the specified array with room to spare (i.e., the array has more elements than this set), the element in the array immediately following the end of the set is set to null. (This is useful in determining the length of this set only if the caller knows that this set does not contain any null elements.)

        If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

        Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

        Suppose x is a set known to contain only strings. The following code can be used to dump the set into a newly allocated array of String: 

          String[] y = x.toArray(new String[0]);
        Note that toArray(new Object[0]) is identical in function to toArray().
        Specified by:
        toArray in interface  Collection<E>
        Specified by:
        toArray in interface  Set<E>
        Overrides:
        toArray in class  AbstractCollection<E>
        Type Parameters:
        T - the runtime type of the array to contain the collection
        Parameters:
        a - the array into which the elements of this set are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
        Returns:
        an array containing all the elements in this set
        Throws:
        ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this set
        NullPointerException - if the specified array is null

      • clear

        public void clear()
        Removes all of the elements from this set. The set will be empty after this call returns.

      • remove

        public boolean remove(Object o)
        Removes the specified element from this set if it is present. More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if this set contains such an element. Returns true if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)
        Specified by:
        remove in interface  Collection<E>
        Specified by:
        remove in interface  Set<E>
        Overrides:
        remove in class  AbstractCollection<E>
        Parameters:
        o - object to be removed from this set, if present
        Returns:
        true if this set contained the specified element

      • add

        public boolean add(E e)
        Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if the set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.
        Specified by:
        add in interface  Collection<E>
        Specified by:
        add in interface  Set<E>
        Overrides:
        add in class  AbstractCollection<E>
        Parameters:
        e - element to be added to this set
        Returns:
        true if this set did not already contain the specified element

      • containsAll

        public boolean containsAll(Collection<?> c)
        Returns true if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns true if it is a subset of this set.
        Specified by:
        containsAll in interface  Collection<E>
        Specified by:
        containsAll in interface  Set<E>
        Overrides:
        containsAll in class  AbstractCollection<E>
        Parameters:
        c - collection to be checked for containment in this set
        Returns:
        true if this set contains all of the elements of the specified collection
        Throws:
        NullPointerException - if the specified collection is null
        See Also:
        contains(Object)

      • addAll

        public boolean addAll(Collection<? extends E> c)
        Adds all of the elements in the specified collection to this set if they're not already present. If the specified collection is also a set, the addAll operation effectively modifies this set so that its value is the union of the two sets. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
        Specified by:
        addAll in interface  Collection<E>
        Specified by:
        addAll in interface  Set<E>
        Overrides:
        addAll in class  AbstractCollection<E>
        Parameters:
        c - collection containing elements to be added to this set
        Returns:
        true if this set changed as a result of the call
        Throws:
        NullPointerException - if the specified collection is null
        See Also:
        add(Object)

      • removeAll

        public boolean removeAll(Collection<?> c)
        Removes from this set all of its elements that are contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.
        Specified by:
        removeAll in interface  Collection<E>
        Specified by:
        removeAll in interface  Set<E>
        Overrides:
        removeAll in class  AbstractSet<E>
        Parameters:
        c - collection containing elements to be removed from this set
        Returns:
        true if this set changed as a result of the call
        Throws:
        ClassCastException - if the class of an element of this set is incompatible with the specified collection (optional)
        NullPointerException - if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
        See Also:
        remove(Object)

      • retainAll

        public boolean retainAll(Collection<?> c)
        Retains only the elements in this set that are contained in the specified collection. In other words, removes from this set all of its elements that are not contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the intersection of the two sets.
        Specified by:
        retainAll in interface  Collection<E>
        Specified by:
        retainAll in interface  Set<E>
        Overrides:
        retainAll in class  AbstractCollection<E>
        Parameters:
        c - collection containing elements to be retained in this set
        Returns:
        true if this set changed as a result of the call
        Throws:
        ClassCastException - if the class of an element of this set is incompatible with the specified collection (optional)
        NullPointerException - if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
        See Also:
        remove(Object)

      • iterator

        public Iterator<E> iterator()
        Returns an iterator over the elements contained in this set in the order in which these elements were added.

        The returned iterator provides a snapshot of the state of the set when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the remove method.

        Specified by:
        iterator in interface  Iterable<E>
        Specified by:
        iterator in interface  Collection<E>
        Specified by:
        iterator in interface  Set<E>
        Specified by:
        iterator in class  AbstractCollection<E>
        Returns:
        an iterator over the elements in this set

      • equals

        public boolean equals(Object o)
        Compares the specified object with this set for equality. Returns true if the specified object is the same object as this object, or if it is also a Set and the elements returned by an iterator over the specified set are the same as the elements returned by an iterator over this set. More formally, the two iterators are considered to return the same elements if they return the same number of elements and for every element e1 returned by the iterator over the specified set, there is an element e2 returned by the iterator over this set such that (e1==null ? e2==null : e1.equals(e2)).
        Specified by:
        equals in interface  Collection<E>
        Specified by:
        equals in interface  Set<E>
        Overrides:
        equals in class  AbstractSet<E>
        Parameters:
        o - object to be compared for equality with this set
        Returns:
        true if the specified object is equal to this set
        See Also:
        Object.hashCode(), HashMap

      • removeIf

        public boolean removeIf(Predicate<? super E> filter)
        Description copied from interface: Collection
        Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
        Specified by:
        removeIf in interface  Collection<E>
        Parameters:
        filter - a predicate which returns true for elements to be removed
        Returns:
        true if any elements were removed

      • forEach

        public void forEach(Consumer<? super E> action)
        Description copied from interface: Iterable
        Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of iteration (if an iteration order is specified). Exceptions thrown by the action are relayed to the caller.
        Specified by:
        forEach in interface  Iterable<E>
        Parameters:
        action - The action to be performed for each element